Video Transcript
A 0.3548-grams sample of a hydrated
salt was strongly heated until a constant mass of 0.3015 grams was obtained. What is the percentage of water of
crystallization in this hydrated salt? Give your answer to two decimal
places.
A hydrated salt is a substance that
contains water of crystallization. Water of crystallization is the
presence of water molecules within the structure of a crystal. For example, the hydrated salt
copper(II) sulfate pentahydrate has the chemical formula CuSO4⋅5H2O. In the formula, we can see that for
each unit of copper(II) sulfate, there are five molecules of water associated with
it.
One way to determine the amount of
water of crystallization in a hydrated salt is to use volatilization gravimetry. Volatilization gravimetry is a mass
analysis method that uses thermal or chemical energy to separate substances in order
to measure the masses of their components. In the question, we are told that
the hydrated salt is heated until a constant mass is achieved. This could be done by heating the
hydrated salt in a crucible using a Bunsen burner. As the hydrated salt is heated,
water molecules are liberated. When the mass of the sample stops
changing, all of the water has been removed. The water that has been removed is
the water of crystallization, and what remains in the crucible is called the
anhydrous salt.
To find the mass of the water lost,
let’s take the mass of the sample before heating and subtract the mass of the sample
after heating. After substituting the values
provided in the problem, we get 0.3548 grams minus 0.3015 grams. Therefore, the mass of water lost
during heating is 0.0533 grams.
Next, to find the percentage of
water of crystallization in the hydrated salt, we need to take the mass of water of
crystallization, which is the mass of water lost, and divide it by the mass of the
hydrated compound before heating. Then, we need to multiply by 100
percent. Let’s divide 0.0533 grams, which is
the mass of water lost, by 0.3548 grams, which is the mass of the sample before
heating, and multiply by 100 percent. The result is approximately 15.0225
percent.
Finally, the question asks us to
give our answer to two decimal places. Therefore, the percentage of water
of crystallization in the hydrated salt is 15.02 percent.
